3898
Further support for the existence of this reversible equilibrium was provided by the Diels-Alder reaction of 2a with N-phenylmaleimide in refluxing toluene. 2)s) adduct, The resulting colorless crystalline ((4 mp 205-207 exhibited spectral properties in unique agreement with structure loa: 6:i:’87.0-7.6 (multiplet, 5 H), 6.0-6.2 (multiplet, 2 H), 3.70 (singlet, 3 H), 2.9-3.6 (multiplet, 4 H), and ~ 1 . (multiplet, 9 2 H). The product is formulated with the imide moiety syn to the double bond in accordance with established precedence. 3 , ~
+
Concerning Azacyclooctatetraene Valence Tautomerism. Preparation of Annelated 7-Azabicyclo[4.2.0]octa-2,4,7-trienes1~ Sir: According to generalized theory, an unsubstituted s-equivalent heterocyclic congener of cyclooctatetraene such as I may theoretically be characterized by dynamic equilibrium with four structurally distinct valence bond tautomers (11-V). In actual fact, only one of the
K-pJ / I1
111
N
v
loa, R = H b, R = CH,
heteroatomic bicyclo[4.2.0]octatrienes is likely to be preferred for reasons such as more advantageous electronic interactions, more favorable conformational In similar fashion, 1,2-dimethyl-l,4-cyclohexadiene environments, and the like. However, placement of (3b) afforded 4b, mp 91-92’,8 in 72% yield. The one or more substituents on the eight-membered ring derived imino ether 5b was quite stable in this instance could affect the original position of equilibrium suffiand afforded a perchlorate salt, mp 139-140°.8 The ciently to cause an alternative bicyclic isomer to be more two-step dehydrogenation of 5b proceeded without stable. The recent synthesis of the azacyclooctatetradifficulty, and 2b was obtained as a stable yellow oil: ene system’ has permitted the first practical assessment bp 47’ (0.35 mm);8 v”,”,”,” 1670 (s), 1640 (m), and 1625 of such a set of electrocyclic changes. cm-I (w); Xisooctane 212 (E 12,900) and 297 mp (E 590); In 2-methoxyazocine ( l a ) and its 3,8-dimethyl analog ;62: 5.72 (singlet, 3 H), 4.95 (broad singlet, 1 H), and (lb), the azacyclooctatetraene form is sufficiently stathree three-proton singlets at 3.63, 1.92, and 1.82. The bilized that the proportion of valence tautomer in the two methyl groups serve to protect the azocine from base-induced aromatization reactions but do not exert an influence which favors the bicyclic valence tautomer 7b.14b However, the facile conversion of 2b to lob, mp 194-195.5°,8 attests to the existence of a OCH, R spectroscopically undetectable equilibrium of the tetla,R=H 2 raene with 7b. b,R = CH3 The striking similarity of the C=C bond stretching frequencies of azocines 2a and 2b with those reported equilibrium is below the spectroscopic detection limit for 117 and the excellent correspondence of the ultra(
